The differential effects of recombinant brain natriuretic peptide, nitroglycerine and dihydralazine on systemic oxygen delivery and gastric mucosal microvascular oxygenation in dogs.

Brain natriuretic peptide has vasodilatory properties and may thus increase splanchnic perfusion and oxygenation. We compared the effects of recombinant brain natriuretic peptide on gastric mucosal microvascular haemoglobin oxygenation (reflectance spectrophotometry) and systemic variables with those of equi-hypotensive doses of two other vasodilators (nitroglycerine and dihydralazine). Chronically instrumented, healthy dogs were randomly allocated to receive on different days, one of the three drugs (nitroglycerine and dihydralazine doses titrated to reduce mean arterial pressure by ∼20%). Brain natriuretic peptide significantly increased gastric mucosal microvascular haemoglobin oxygenation selectively, i.e. without concomitant haemodynamic effects. In contrast, the other vasodilators either did not increase gastric mucosal microvascular haemoglobin oxygenation at all (nitroglycerine), or did so only with marked increases in other systemic haemodynamic variables (dihydralazine). Our data suggest a potential role of recombinant brain natriuretic peptide selectively for increasing microvascular mucosal oxygenation. Studies are required to extend these findings to the clinical setting.

Effects of dihydralazine on renal water and aquaporin-2 excretion in humans.

OBJECTIVE: Dihydralazine is a vasodilator that lowers blood pressure, but often also leads to significant water and sodium retention. To characterize the effect of dihydralazine on renal sodium and water handling, we tested the hypothesis that dihydralazine causes water retention parallel with an increase in urinary excretion of aquaporin-2 (u-AQP2) in healthy humans. MATERIAL AND METHODS: The effect of intravenous infusion of dihydralazine in three doses (3.125 mg, 6.250 mg and 9.375 mg) on urinary AQP2, water and sodium excretion, heart rate (HR), blood pressure (BP) and vasoactive hormones was measured in a randomized, placebo-controlled, double-blind, crossover study in 17 healthy subjects. Glomerular filtration rate (GFR) and renal tubular function were determined with the continuous infusion clearance technique and vasoactive hormones with radioimmunoassays. RESULTS: Dihydralazine compared to placebo had no impact of u-AQP2 (effect of dihydralazine versus placebo +/-SE) (-0.074+/-0.048 ng/min versus -0.015+/-0.034 ng/min; p = 0.42), despite significant reductions in urine output and free water clearance after 9.375 mg of dihydralazine. Dihydralazine significantly lowered BP and increased HR, plasma levels of angiotensin II and (except after 3.125 mg) atrial natriuretic peptide, while plasma levels of vasopressin, GFR and fractional excretions of sodium and lithium were not significantly changed. CONCLUSIONS: These findings suggest that dihydralazine increases water re-absorption in the distal tubules, independently of vasopressin and of sodium re-absorption. Furthermore, our study does not support an effect of the sympathetic nervous system, the renin-angiotensin system and the natriuretic peptide system on u-AQP2 regulation.

Evaluation of Strategies for the Assessment of Drug-Drug Interactions Involving Cytochrome P450 Enzymes.

BACKGROUND AND OBJECTIVES: Drug-drug interactions (DDIs) can occur when one drug alters the metabolism of another drug. Drug metabolism mediated by cytochrome P450 enzymes (CYPs) is responsible for the majority of metabolism of known drugs and inhibition of CYP enzymes is a well-known cause of DDIs. In the current study, the use of various human liver microsomes (HLM)-based methods to determine occurrence of CYP-mediated metabolism-dependent inhibition (MDI) and possible follow-up studies were evaluated. METHODS: Human CYP inhibition was studied using the following methodologies: direct inhibition and (non-diluted) IC50-shift assays, a ferricyanide-based reversibility assay, a spectrophotometric metabolic intermediate complex (MIC) assay, and recording of reduced carbon monoxide (CO)-difference spectra. HLM incubations in the presence and absence of NADPH and glutathione (GSH) were performed to study the possible formation of CYP-dependent GSH adducts. HLM incubations with the radiolabeled inhibitors mifepristone and paroxetine were performed to study CYP-mediated covalent binding. RESULTS: Dihydralazine and furafylline displayed irreversible MDI of CYP1A2. Paroxetine displayed both quasi-irreversible and irreversible MDI of CYP2D6, formation of CYP-dependent GSH adducts was observed, while CYP-mediated covalent binding occurred which was decreased in the presence of GSH. Mifepristone displayed irreversible MDI of CYP3A4, formation of CYP-dependent GSH adducts was observed, while CYP-mediated covalent binding occurred which was decreased in the presence of GSH. Troleandomycin and verapamil displayed quasi-irreversible MDI of CYP3A4; MIC formation was observed, while no formation of CYP-dependent GSH adducts occurred. CONCLUSIONS: This study gives a representative overview of current methodologies that can be used to study CYP inhibition. The here presented strategy can be applied as a tool during risk evaluation of CYP-mediated DDIs.

Estimation of forearm arterial compliance in normal and hypertensive men from simultaneous pressure and flow measurements in the brachial artery, using a pulsed Doppler device and a first-order arterial model during diastole.

Simultaneous brachial artery pressure and blood flow measurements were made in 45 men. Blood flow was evaluated by means of a pulsed Doppler device with a double transducer probe. From analysis of the pressure-flow curves during diastole, forearm arterial compliance (FAC) was determined by using the model of the forearm arterial tree as a system of tubes, each with a storage capacitance, in series with the arteriolar resistances vessels. The value of FAC for seven normal subjects, aged 44 +/- 3 (mean +/- SEM) years, was between 0.78 and 1.73 X 10(-10) m5 . N-1. By comparison, a 30% reduction in FAC was observed in 38 men of the same age with essential hypertension, which was similar whether the intra-arterial diastolic pressure was above or below 90 mmHg. In the more severe group (Intra arterial diastolic pressure greater than 90 mmHg), the reduced FAC was associated with a significant increase in brachial artery diameter; after administration of dihydralazine, blood pressure and arterial diameter returned to normal but FAC remained diminished. The study is the first to evaluate FAC in intact men. The reduced FAC in hypertension is independent of blood pressure "per se" but may reflect adaptive change in the walls of the large arteries. In the more severe hypertension, arterial calibre was increased; this could be a mechanism which could prevent FAC from decreasing further with chronic elevation of blood pressure.

Blood pressure response and renin release following 4 days of treatment with dihydralazine and urapidil in conscious dogs.

The magnitude and the persistence of blood pressure reduction by dihydralazine and urapidil were investigated following treatment over a period of 4 days. The experiments were performed on six normotensive dogs, trained to submit to puncture of the femoral artery and to stand quietly in a special frame. The first-dose effects of orally administered dihydralazine (1.42, 7.1 mg X kg-1) and urapidil (2.0, 10.0 mg X kg-1) on heart rate, arterial blood pressure and plasma renin activity (PRA) were compared with the effects of the substances after 4 days of treatment. Both compounds caused a dose-dependent decrease in blood pressure but in contrast to urapidil the effect of dihydralazine was accompanied by large increases in heart rate and PRA. Dose-dependent tolerance to dihydralazine but not to urapidil was observed after treatment over only 4 days. However, basal blood pressure was significantly lowered after 4 days of treatment with urapidil at the high dosage and no further reduction was achieved on the fifth day. The importance of persistent counterregulation in the development of tolerance to the antihypertensive effect of dihydralazine is discussed.

Peripheral large arteries and the response to antihypertensive treatment.

Since systolic pressure is governed by the rate of ventricular ejection and the rigidity of the aortic wall, antihypertensive agents may have different effects on systolic and diastolic pressure. Despite an adequate decrease in diastolic pressure, systolic pressure may remain elevated due to structural alterations of large arteries. In the present study, a procedure is described to distinguish the dilation of small and large arteries. The former is evaluated from the calculation of forearm resistance and the latter from the determination of the arterial diameter of the brachial artery, using a bidimensional pulsed Doppler system. Nitroglycerin dilates the brachial artery, with no change in forearm resistance. Dihydralazine reduces the diameter of the brachial artery but decreases forearm resistance. Only calcium and converting-enzyme inhibitors dilate both small and large arteries and cause an increase in brachial blood flow.

Differential gene expression of renin and angiotensinogen in the TGR(mREN-2)27 transgenic rat.

Transgenic rats carrying the murine Ren-2 gene represent a monogenetic model of hypertension characterized by low plasma renin and high extrarenal expression of the transgene. The hypothesis has been raised that stimulated local reninangiotensin systems may be responsible for the development of hypertension in this model. This study analyzes the effects of the converting enzyme inhibitor lisinopril, which specifically interferes with the renin-angiotensin system, and the direct vasodilator dihydralazine on the renal and extrarenal expression of renin and angiotensinogen. A comparison of gene expression between heterozygous and homozygous transgenic and normal Sprague-Dawley rats was also performed. We demonstrate high sensitivity of blood pressure toward converting enzyme inhibition in transgenic TGR(mREN-2)27 rats. In the kidney, expression of the transgene and the endogenous renin gene increased, suggesting that both are modulated by lisinopril in a similar manner. On the other hand, blood pressure reduction by dihydralazine did not abolish renal renin suppression in transgenic rats, indicating that mechanisms different from direct effects of blood pressure account for renin suppression. Homozygosity for the transgene led to increased Ren-2 expression and higher blood pressure and had opposite effects on angiotensinogen expression compared with heterozygous rats. Cardiac hypertrophy was reduced by lisinopril but not dihydralazine and was positively correlated with cardiac angiotensinogen expression. Increased angiotensin II in the adrenal gland of TGR(mREN-2)27 rats, which overexpresses the transgene, provides evidence that this leads to enhanced generation of tissue angiotensin II. We conclude that expression of the mouse transgene, the endogenous rat renin gene, and the angiotensinogen gene is subject to differential tissue-specific regulation. Reversal of cardiovascular damage with the converting enzyme inhibitor but not dihydralazine suggests that angiotensin II generated locally may be involved in the pathogenesis of hypertension and structural changes in TGR(mREN-2)27 rats.

Dynamic responses to intravenous urapidil and dihydralazine in normal subjects.

Hemodynamic responses after urapidil were compared with those after dihydralazine in placebo-controlled, double-blind studies after cumulative intravenous doses. We recorded heart rate, blood pressure, systolic time intervals corrected for heart rate (electromechanical systole and preejection period), electrical impedance cardiography [(dZ/dt)/RZ index and mean electrical thorax impedance], and M-mode echocardiogram (end-systolic and -diastolic diameters, end-systolic wall stress, fractional shortening, and cardiac output). Both drugs induced dose-dependent reductions in total peripheral resistance, which resulted in reduction in left ventricular end-systolic wall stress and increases in heart rate (limited at +10 bpm with urapidil), fractional shortening, cardiac output, and the (dZ/dt)/RZ index. With each drug, diastolic blood pressure fell by 5 mm Hg, the corrected preejection period shortened (dihydralazine greater than urapidil), the corrected electromechanical systole did not change, and mean electrical thorax impedance rose with urapidil. The spectrum of effects indicates that both drugs reduce left ventricular afterload, thereby increasing left ventricular pump performance. Urapidil also exerts some preload reduction.

A comparison between the haemodynamic effects of oral nifedipine and intravenous dihydralazine in patients with severe pre-eclampsia.

OBJECTIVE: To compare the effects of a single oral dose of nifedipine with those of intravenous dihydralazine on central haemodynamics in pregnant women with severe pre-eclampsia. DESIGN: A prospective comparative study. SETTING: The High Risk Obstetric Unit, University Hospital Rotterdam Dijkzigt, Rotterdam. SUBJECTS: Twenty patients with severe pre-eclampsia between 27 and 35 weeks gestation with normal cardiac filling pressures and without fetal distress. INTERVENTIONS: A pulmonary artery thermodilution catheter and a radial artery line were placed. Ten patients chewed a 10-mg capsule of nifedipine and 10 patients received dihydralazine by intravenous infusion at a rate of 1-3 mg/h. Arterial pressures, heart rate, cardiac output and pulmonary capillary wedge pressure were determined before and after drug administration. Fetal condition was continuously monitored by cardiotocography. RESULTS: The reduction in arterial blood pressure obtained with both drugs was similar, and was associated with a similar rise in heart rate and cardiac output and a similar reduction in systemic vascular resistance. Pulmonary capillary wedge pressures decreased significantly less with nifedipine than with dihydralazine. Signs of fetal distress occurred in none of the nifedipine-treated patients, but in five of the patients treated with dihydralazine. CONCLUSION: From the haemodynamic viewpoint nifedipine seems to be a useful agent in the treatment of hypertensive emergencies in pregnancy.

6-Aryl-4,5-dihydro-3(2H)-pyridazinones. A new class of compounds with platelet aggregation inhibiting and hypotensive activities.

This paper reports on the synthesis and pharmacological activity of 6-aryl-4,5-dihydro-3(2H)-pyridazinone derivatives. The compounds exhibit an aggregation inhibiting action on human platelets in vitro and on rat platelets under ex vivo conditions, as well as a hypotensive action on rats. The strongest pharmacological effects were found with dihydropyridazinones, which have a 6-[p-[(chloroalkanoyl)amino]phenyl] substituent, together with a methyl group in the 5-position. The antiaggregation activity of compounds of this type is in vitro up to 16000 times and ex vivo up to 370 times greater than that of acetylsalicylic acid; the hypotensive action is up to 40 times as great as that of the comparative substance dihydralazine.

Neuromodulatory effects of the renin-angiotensin system on the cat electroretinogram.

PURPOSE: Angiotensin-converting-enzyme (ACE) catalyses the formation of angiotensin II (ANGII), which presumably acts as a central neurotransmitter/modulator. ANGII-related effects have also been observed in the retina. Present in vivo experiments were designed to investigate further ANGII-related effects on retinal neuromodulation. METHODS: In 12 anesthetized cats, electroretinographic measurements were carried out in the dark-adapted state using corneal contact lens electrodes and a Ganzfeld stimulator. Quinapril was used to inhibit ACE. RESULTS: Reducing ANGII-concentration increased sensitivity (0.5 log units) and gain (50%) of the rod b-wave amplitude. The b-wave implicit time was stimulus dependent, shortening at high intensities. The scotopic threshold response and the oscillatory potentials were also influenced by ACE inhibition. However, a-wave and 30 Hz flicker remained unaffected. Effects of Quinapril on ERG-amplitudes were reversed by subsequent ANGII administration, except for the implicit time of the b-wave and scotopic threshold response. CONCLUSIONS: Although these results are accompanied by alterations in systemic blood pressure, several findings support the evidence that the renin-angiotensin system might have a neurophysiologic effect on retinal neurons outside the vascular system. These results are in accordance with immunohistochemical data found by others that point to angiotensinergic cell involvement and thereby further support the concept of angiotensinergic processes in the inner retina from a functional point of view.

Blood pressure changes in spontaneously hypertensive rats correlate with aortic prostacyclin formation.

1 The relationship between the blood pressure fall, induced by antihypertensive drugs or bleeding, and the formation of prostacyclin (PGI2)-like activity in the thoracic aorta of spontaneously hypertensive rats has been investigated. Inhibition of ADP-induced platelet aggregation was used to assess PGI2-like activity. 2 The decreases in blood pressure produced by clonidine, dihydralazine and prazosin were associated with increases of PGI2-like activity of 50-80%. The increase in PGI2-like activity correlated well with the blood pressure decrease, independently of the mechanism of the fall in blood pressure.

Compared myocardial and vascular effects of captopril and dihydralazine during hypertension development in spontaneously hypertensive rats.

When administered to young spontaneously hypertensive rats (SHRs), dihydralazine (25 mg kg-1, daily) and captopril (100 mg kg-1, daily) prevent with the same efficacy genetic hypertension development (GHD). Dihydralazine treatment increased vascular mesenteric compliance, as shown by a significant decrease in the stiffness of the vessels (-27%), and induced slight reductions in contractility (-12%) and in wall to lumen (W/L) ratio (-15%). After treatment withdrawal, all these parameters returned to control values within 7 weeks, as did blood pressure. Captopril treatment also strongly increased the mesenteric vessels compliance, vessel stiffness being decreased by 16%, and reduced their contractility (-15%) and their W/L ratio (-30%). These effects as well as those exerted on blood pressure persisted up to 7 weeks after treatment ceased although there was a slight trend to a progressive reduction in the intensity of both phenomena. These experiments show that captopril but not dihydralazine has a long-lasting effect in opposing the functional and morphological vascular alterations occurring during GHD in SHRs and this phenomenon probably contributes to a large extent to the sustained preventive effects of the drug against GHD.

Increased activity of the GABAergic system in selected brain areas after chronic propranolol treatment in spontaneously hypertensive rats.

The influence of chronically administered propranolol on the functional state of the gamma-aminobutyric acid-ergic (GABAergic) system in spontaneously hypertensive rats was studied and compared with the effect of dihydralazine. GABA content, synthesis and turnover rate in selected brain areas were assessed. Hypotensive activity of propranolol and dihydralazine after injection of GABA antagonist pictrotoxin was examined in acute experiment. Prolonged administration of propranolol increased GABA content, synthesis and turnover rate in the hypothalamus and the pons-medulla. After chronic injections of dihydralazine there was no change in GABA indices. Antihypertensive activity of dihydralazine in picrotoxin-treated animals remained unchanged. On the contrary, picrotoxin suppressed the propranolol-induced decrease in blood pressure. Our results indicate that propranolol increases GABAergic system activity. Therefore, we conclude that down-regulation of the GABAergic system in hypertension may be compensated by the regulatory action of propranolol.

Acute treatment of hypertension increases infarct sizes in spontaneously hypertensive rats.

We studied the effect of dihydralazine treatment of hypertension in spontaneously hypertensive stroke-prone rats in a model of permanent focal cerebral ischemia (stroke). After occlusion of the middle cerebral artery systemic arterial pressure (SAP) was lowered with a computer controlled infusion device from 163 to 135 or 117 mm Hg for 24h. In the control group SAP was not manipulated. Reduction of SAP to normotension (117 mm Hg) significantly worsened outcome and increased infarct volume measured 7 days after induction of ischemia, whereas a mild reduction of SAP (to 137 mm Hg) had no statistically significant effect on outcome or infarct volume. We conclude that pharmacological treatment of hypertension may negatively affect neurological outcome and infarct volume in a rat stroke model.

Differential effects of long-term hypoxia on norepinephrine turnover in brain stem cell groups.

The influence of long-term hypoxia on noradrenergic cell groups in the brain stem was assessed by estimating the changes in norepinephrine (NE) turnover in A1, A2 (subdivided into anterior and posterior parts), A5, and A6 groups in rats exposed to hypoxia (10% O2-90% N2) for 14 days. The NE turnover was decreased in A5 and A6 groups but failed to change significantly in A1. The NE turnover was increased in the posterior part of A2 and remained unaltered in the anterior part. In normoxic rats, the hypotensive drug dihydralazine induced a reverse effect, namely increased NE turnover in anterior A2 and no change in posterior A2. The neurochemical responses to hypoxia were abolished by transection of carotid sinus nerves. The results show that long-term hypoxia exerts differential effects on the noradrenergic cell groups located in the brain stem. Peripheral chemosensory inputs control the hypoxia-induced noradrenergic alterations. The A2 cell group displays a functional subdivision: the posterior part is influenced by peripheral chemosensory inputs, whereas the anterior part may be concerned with barosensitivity.

Effects of dihydralazine infusion on the fetoplacental blood flow and maternal prostanoids.

The hemodynamic effects of intravenously infused dihydralazine (incremental doses up to 125 micrograms per minute during 60 minutes) were studied in ten women with acute or superimposed severe preeclampsia. The intervillous and umbilical vein blood flow were measured before and during dihydralazine infusion with 133Xenon method and with a combination of real-time and Doppler ultrasonic equipment, respectively. Maternal blood pressure decreased and pulse rate increased during the infusion. Dihydralazine did not change the intervillous blood flow but it increased the blood flow in umbilical vein. No effect on the 6-ketoprostaglandin F1 alpha in maternal plasma and urine or thromboxane B2 in maternal serum was observed. The results indicate that dihydralazine affects the placental and fetal circulations differently.

Effects of clonidine and dihydralazine on atrial natriuretic factor and cGMP in humans.

The effects of a 1-wk treatment with clonidine (75 micrograms/day twice a day) and dihydralazine (25 mg/day twice a day) on base-line levels of plasma atrial natriuretic factor (ANF) and plasma and urinary guanosine 3',5'-cyclic monophosphate (cGMP) and their changes by acute saline infusion (2 liters) in eight normal subjects were evaluated. Basal ANF was decreased to 65% in the clonidine group compared with both the control and dihydralazine groups. Volume loading increased plasma ANF levels by 30-40% of base-line values in the control and the dihydralazine groups and by 15% in the clonidine group. Basal plasma and urinary cGMP levels were raised by 30 and 90% in the dihydralazine group compared with both other groups. Volume loading increased plasma cGMP levels by 40% in the control and clonidine-treated groups and by 25% in the dihydralazine-treated group. It is concluded that ANF may contribute to hemodynamic effects of clonidine but not to those of dihydralazine. Dihydralazine increases plasma and urinary cGMP, supposedly by direct activation of the soluble guanylate cyclase.

Regional specificity of long-term regulation of tyrosine hydroxylase in some catecholaminergic rat brainstem areas. II. Effect of a chronic dihydralazine treatment.

Dihydralazine, which is used in the treatment of hypertension, causes a long-lasting hypotensive action by a direct vasodilator effect on arteriolar smooth muscle. The present study was carried out to investigate the effect of a daily single injection of dihydralazine (20 mg/kg, s.c.) for 14 days on the tyrosine hydroxylase (TH) protein quantity in some catecholaminergic rat brainstem areas such as the dorsomedial medulla (DMM), the ventrolateral medulla (VLM) and the locus coeruleus (LC). This study demonstrates that the dihydralazine produced (1) an 85% increase in TH protein quantity exclusively in the rostral part of DMM, (2) a 58% increase of TH protein content exclusively in the rostral part of the LC, and (3) a 37% increase of the TH protein quantity in VLM catecholaminergic area. To determine whether the increase in TH protein quantity could be related to a change in norepinephrine (NE) content, the rate constant of disappearance (k) of NE was measured in the catecholaminergic regions of the same rats treated with dihydralazine. Our results show that dihydralazine causes an increase of the TH protein, in addition to an elevation of NE content, within the subpopulations of catecholaminergic structures. These data suggest a selective response of the TH regulation to dihydralazine within the rostral DMM area which receives barosensory inputs.

Similar increase in circulating renin after equihypotensive doses of nitroprusside, dihydralazine or isradipine in conscious rabbits.

Calcium entry blockers but not nitroprusside sodium (NPS) or dihydralazine are known to enhance renin release in vitro. Isradipine (PN200-110), NPS or dihydralazine were infused at 15 min intervals into groups of 6 conscious, chronically instrumented rabbits each, at 3 doses carefully matched to elicit comparable falls in blood pressure. Plasma renin activity increased similarly with all treatments. In intact animals, the magnitude of the pressure decrease appears to be more important for renin release than the mechanism of vasodilation and a typical calcium entry blocker was indistinguishable from other vasodilators.